On the manifold thing its pretty interesting if you can find a picture of the stock design UEL and consider that the exhaust gasses from cylinders on the passenger side of the engine have about 4 or 5 times the length to travel than the drivers side making it less efficient. I had EL on my 06 wrx and the earlier spool was definately noticeable imo but obivously loss of rumble sound.

Also factor in that the MY06-07 wrx oil cooler sits on an angled sandwhich plate of sorts which angles back into the runners of EL headers and makes it impossible to fit without changing the oil cooler. Perrin put together a kit of oem parts thats gets around it but cost me an additional $400.

I could be wrong but i don't think the aftermarket UEL would be worth the $5-700, probably better mods to be had for that sort of money. A catless up pipe that bolts into stock manifold could be a go for a bit less restriction.

And can remember reading that the split dump pipe is the way to go, somthing about the turbine and wastegate gasses combining together in the bell mouth not being good for x reason.